In a CCD solid state imaging device or a MOS (metal-oxide semiconductor) solid state imaging device, micro lenses are formed to improve the efficiency of focusing light to photosensitive parts, to thereby increase the amount of incident light upon pixels and improve the sensitivity of the device. The micro lenses corresponding to the pixels in one-to-one correspondence are arranged in, e.g., a matrix pattern. To ameliorate the sensitivity of the CCD or CMOS (complimentary metal-oxide-semiconductor) sensor, the micro lenses are required to be formed to have large areas to receive more light at their condensing points. Accordingly, a distance between neighboring micro lenses is required to be small. Specifically, as shown in FIGS. 10A and 10B a distance D1 between micro lenses 100 in horizontally or vertically parallel positions to each other (hereinafter simply referred to as a horizontal or vertical distance D1), and a distance D2 between micro lenses in diagonal positions to each other (hereinafter, simply referred to as a diagonal distance D2) need to be reduced.
Meanwhile, referring to FIG. 10C, used to form such micro lenses 3 is a wafer W on which a mask layer 103 is formed on an underlayer 101 via a lens material layer 102 made up of, e.g., an organic material, wherein the underlayer 101 is provided with a photosensitive part and a conductive film. The mask layer 103 is formed to have a lens shape as shown in FIG. 10C. By etching the mask layer 103 and the lens material layer 102 by a plasma of a processing gas including, e.g., a C-H-F based gas and an O2 gas, the lens shape of the mask layer 103 is transcribed to the lens material layer 102, so that the micro lenses 100 are obtained.
When forming the mask layer 103 in the lens shape, the organic mask layer 103 is subjected to softening by a heat treatment after an exposure process. However, if the lenses are made to contact each other by the softening, the lens shape would be collapsed due to a surface tension therebetween. For the reason, the lenses are arranged at the horizontal or vertical distance D1 of, e.g., 0.5 to 0.2 μm, and thus the diagonal distance D2 between the lenses is set to be, e.g., 1 μm. Accordingly, intervals corresponding to D1 and D2 are also formed between the micro lenses 100 transcribed to the lens material layer 102.
Japanese Patent Laid-open Application No. 2005-101232 (paragraphs (0010) and (0015)) and Japanese Patent Laid-open Application No.H10-148704 (paragraphs (0049) and (0059)) disclose techniques for reducing the intervals between the micro lenses 100. These techniques attempt to reduce the intervals between the micro lenses 100 by accumulating deposits on sidewall portions of the lenses formed on the mask layer 103 and by etching the mask layer 103 and the lens material 102 simultaneously with the deposition. However, from the inspection of the inventor of the present invention, it is found that though the D1 can be reduced to zero by those techniques, the speed with which the D2 is narrowed (i.e., the decreasing rate of the D2) is very slow. Accordingly, though it is possible to reduce the D1 to zero within a processing time set by considering a throughput of a production line, the D2 still remains great, thus impeding the improvement of the sensitivity of the solid state imaging device.